The present invention relates to a method of remotely polling or interrogating electronic tags (also known as radio tags) from a station. It also relates to a station for remotely polling electronic tags, notably for implementing the method.
The invention further provides an electronic tag having means for communicating with the station for receiving polling signals or interrogations from the station and for sending responses back to the station, notably for implementing the method.
Further, the invention also relates to a combination of such a station and such electronic tags.
The invention generally relates to the field of polling electronic tags remotely from a station or exit gate or the like. Numerous applications are possible, in widely-ranging fields. The following can be given as examples:
checking, checking out and/or verifying electronically-tagged articles purchased by supermarkets customers; PA1 counting of tagged objects for stock management in a factory or industrial plant; PA1 recognizing and orienting tagged objects such as, for example, suitcases in an airport; PA1 recognizing the passage of objects fitted with tags, such as for example automobiles at a pay station, and exchange of information with such objects; PA1 in systems for restricting access or for monitoring access of persons carrying an electronic tag to an installation or site, etc. PA1 a resonant circuit with an inductor and a tuning capacitor; PA1 means for exciting said resonant circuit with an excitation alternating voltage; PA1 means for blocking said excitation alternating voltage at one half wave, and PA1 means for maintaining a permanent current in said inductor.
Other applications are possible.
In the remainder of this specification, the term "tag" will simply be used to designate an electronic tag, independently of its physical support or use.
The term "gate" or "station" will generally be used for the inquiry station from which tags are polled; the physical form of this station is obviously not limited to the physical structure of a frame or gate.
By the terms interrogation, polling, or probing, we mean a station calling up, interrogating or polling tags in order to, if necessary, exchange information with them.
Systems for remotely polling electronic tags already exist.
In the case of simultaneous interrogation of several tags, the problem arises from the tags replying simultaneously or in overlapping fashion, which we shall call below "collision". Generally speaking, prior art documents propose that the tags send their responses in a given format designed to be recognised by the station; in some documents, it is proposed that the response from the tags should depend on their code. The station can then check the validity of the response it has received, in order to detect collisions.
The following documents disclose examples of solutions to the problem of collision detection and avoidance: European Patent Applications EP-A-0,495,708, EP-A-0,161,779, EP-A-0,472,472 or British Patent Application GB-A-2,116,808, which disclose ways of detecting collisions and avoiding them.
EP-A-0,473,569 discloses a system of this type in which tags send a message consisting of their code, along with an error correcting code for collision detection purposes. Synchronization of responses from the various tags is also disclosed. In U.S. Pat. No. 5,264,854, tags send their responses at random instants in a time window, for limiting the probability of collision.
These solutions suffer from disadvantages: the presence of an error-correcting code, even of a very summary nature, requires fairly long messages. This has the effect of increasing total tag polling time. Furthermore, recourse to using responses at random points in time also lengthens the duration of polling.
The use of tags that do not have their own power supply, and which employ power transmitted by the station as their source of power has also been disclosed; in such a case where the tags are remotely powered, various systems have been proposed for the sending of responses by the tags.
It is possible for the tag to send its response at a radio frequency which is distinctly higher than the remote polling frequency. In this case, it is easy to separate, using filtering, the remote polling frequency from the frequency transmitted in return in the station's receiving coil. However, in order to get a stable and accurate sending frequency, a quartz oscillator is needed or a frequency multiplier or an oscillator with a controlled frequency, using the remote polling frequency as a reference for said control. Such methods are expensive in component costs or in semi-conductor chip power consumption.
The tag can also respond at the same frequency as the remote polling frequency; this is the case, for example, in GB-A-2,116,808. In this case, in order to detect the signal sent by tags, the station's inductor needs to be shut off. This means there is no longer any reference frequency available, or otherwise, one is confronted with the problem of the expense and bulk of providing a quartz oscillator in each tag. Simply using the coil's own resonant frequency or an RC oscillator would lead to considerable frequency inaccuracy, necessitating the use of a "panoramic" receiver cable of detecting responses in several tens of partially-overlapping channels covering all possible ranges of frequency drift. Such a receiver would be complex and expensive.
It has also been proposed to short-circuit the tag's coil as a way of sending back responses from tags, thereby generating a slight variation in current in the inductor. This method which is for example disclosed in EP-A-0,2,42,906 is simple and does not require a complicated circuit in the tag. However, variation in current is undetectable when the inductor covers a large area, in other words when the useful volume exceeds some hundred liters.
WO-A-84/03566 proposes sending the response from the tag at a frequency n.f..sub.S, where f.sub.S is the station's sending frequency. Values of n of 1,2 or 3 are proposed or, further in another embodiment, 1/2, 1/3, 1/4, etc. The device proposed in the tag comprises separate sending and receiving coils.
EP-A-0,006,691 discloses a tag using a single send-receive coil in which sub-harmonics of the station sending frequency are obtained by means of a switching device situated between the coil and a tuning capacitor. Another problem is that of the position of a tag within the electromagnetic polling field broadcast by the station. Indeed in some applications, tags can present themselves in the polling field with a random spatial orientation. If a receive coil of a tag is located in a plane parallel to the station's inductive field, no flux will pass through the coil and, in the absence of an induced voltage, the tag cannot reply.
To resolve this problem, in EP-A-0,496,609, it is proposed to employ two sending antennae in the form of frame antennae, situated in perpendicular planes, and supplied with signals in phase quadrature, in order to establish a rotating polling field.
This solution is not satisfactory: the induced field remains in this case in a plane orthogonal to the two frames leaving open the possibility of tags situated in this plane not being detected.
It has also been proposed in EP-A-0,496,609, to employ a third sending antenna in the form of a frame, located in a plane orthogonal to the two other antennae planes and to feed the three antennae with signals in quadrature.
This solution could be satisfactory in the case of one single tag. The effect would be moreover the same if one were to employ three antennae in three different planes, exciting them successively pairwise with signals in quadrature. More generally, the use of two or three antennae with signals in quadrature is not satisfactory.